/**
* Allows you to check if the migration is effectively installed (entirely optional)
*
* This is checked when a migration is installed. If true is returned, the migration will be set as
* installed without performing the database changes.
* This function is intended to help moving to migrations from a previous database updater, where some
* migrations may have been installed already even though they are not yet listed in the migrations table.
*
* @return bool True if this migration is installed, False if this migration is not installed (checked on install)
*/

Oleg wrote:What does that offer that could not have been achieved with recording which migrations have been run without actually running them?

The way I understand the documentation is that this is exactly what effectively installed is used for. To figure out which migrations need to be on the record without having been run based on the current state of the database. Once the migration system has been installed as part of the update this information is no longer necessary, as the records of run migrations are then sufficient.